A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Between the reticle and the substrate is disposed a projection system for imaging the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of irradiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
Generally, imaging of Phase Shift Mask (PSM) structures uses an on-axis pupil filling with a low pupil filling factor (i.e. σ) of less than 0.4. This allows very small pitches and/or critical dimensions approaching k1=0.25 to be used. An important feature of these prior art un-polarized on-axis settings in combination with PSM structures is that it allows imaging of dense features in many orientations during the same exposure. The resolution that can be printed by a lithographic machine is often expressed by the k1 value. The k1 value relates the resolution R that can be printed to the wavelength λ and the numerical aperture NA of the machine by R=k1λ/NA.
However, when the pitch of the PSM features become smaller, such as less than the wavelength divided by twice the numerical aperture, then a portion of the first order (of diffracted radiation) falls outside the numerical aperture (i.e. NA). This has the disadvantage of rapidly decreasing the image contrast.